1. Field of the Invention
This invention relates to a thin film transistor (TFT) liquid crystal display, and more particularly, to a discharging apparatus for a liquid crystal display for substantially reducing a residual image upon power-off.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) of active matrix driving system uses thin film transistors (TFT's) as switching devices to display a natural-like moving picture. Since such an LCD device can be made smaller than an existing Brown tube, it has been widely used as a monitor for personal or notebook computers, as well as in office automation equipment, such as copy machines, facsimile machines, and the like; and in portable equipment such as cellular phones and pagers, and the like.
An active matrix LCD device displays a picture by controlling light transmissivity within pixel cells of the liquid crystal device in accordance with an electric field applied to the liquid crystal of each cell. However, an existing problem of active matrix LCD's is that a voltage across a liquid crystal cell slowly decreases just after power to the device is turned off. This slow voltage decrease causes an undesirable residual image in the display after the display device is turned off (after power-off).
FIG. 1 shows one method currently used for overcoming the problem of residual LCD image after power-off. As shown in FIG. 1, a liquid crystal display panel is provided with discharge circuits 12 to eliminate LCD residual image when the device is powered off. The LCD panel includes a TFT arranged at each intersection between a gate line GL and a data line DL. Each of the TFT's includes a liquid crystal cell Clc connected between its drain and common voltage source Vcom. An auxiliary capacitor Cst is connected in parallel to each of the liquid crystal cell Clc, and each of the discharge circuits 12 is connected to one of the gate lines GL.
To operate a pixel defined at an intersection of a gate line GL and data line DL, a gate signal, e.g., a gate high voltage and a gate low voltage from a gate driver (not shown), may be applied to the gate line GL. At the same time, data voltage from a data driver (not shown) may be applied to the data line DL. The TFT is turned on when a gate high voltage is applied to the gate line GL. Consequently, the liquid crystal cell Clc is charged by the voltage difference between the data voltage from the data line DL and the common voltage Vcom. The liquid crystal cell Clc maintains voltage charge during a period when a gate low voltage Vgl is applied to the gate line GL, and the auxiliary capacitor Cst allows stable maintenance of the voltage charged in the liquid crystal cell Clc.
The discharge circuit 12 includes a PMOS transistor M1 for defining a discharge path upon power-off, a diode D1, and a capacitor C2 connected to the PMOS transistor M1. The capacitor C1 is connected between a voltage supply line VDDL and a gate terminal of the PMOS transistor M1. The diode D1 is connected between the gate of the PMOS transistor M1 and the source of the PMOS transistor connected to a ground line GNDL. The diode D1 and the capacitor C1 sense power-on/off by a supply voltage VDD from the voltage supply line VDDL to turn off or on the PMOS transistor M1. Upon power-off, the PMOS transistor M1 is turned on to define a discharge path for the liquid crystal cell Clc and the auxiliary capacitor Cst. Thus, upon power-off, a fast discharge of the liquid crystal cell Clc and the auxiliary capacitor Cst in the LCD eliminates a residual image on the LCD.
However, the conventional power-off discharge circuit has a drawback in that the liquid crystal display panel has a complicated structure since the discharge circuit 12 is provided on the liquid crystal display panel for each gate line GL. Moreover, since both the voltage supply line VDDL and the ground line GNDL for each discharge circuit 12 are formed on the liquid crystal display panel, the corresponding increase in the number of electrode lines complicates the liquid crystal panel display structure and adds to manufacturing complexity and costs. Thus, there remains a need in the art for a simple, low cost liquid crystal display structure that is capable of reducing residual image upon power-off.